ADHARA (Epsilon Canis Majoris). The names of all the first magnitude stars ring clearly to us;
even the names of those in the southern hemisphere are well known.
Adhara has escaped the fame it deserves. Though better known as
Epsilon Canis Majoris (using the fifth letter in the Greek alphabet), it's actually the second
brightest star in its constellation (after
Sirius), and helps form the western leg
of Canis Major, the larger dog. Look just
below Sirius to find an outstanding triangle of bright stars.
Adhara is at the lower right. Bayer thus seems to have used
position in the constellation rather than brightness. The ancient
Arabs referred to this small three-star pattern as "The Virgins,"
to which the name "Adhara" actually refers (Adhara the westernmost
of the three). No one knows why the name was given or who they
were. Adhara has an apparent magnitude of 1.50, and therefore is
sometimes referred to as the last of the first magnitude stars or
as the brightest of the second magnitude depending on where one
wants to draw the line. The latter view and the star's rather
southern position in the sky has led to its being somewhat ignored.
In fact, Adhara, a class B (B2) bright giant, is quite the magnificent
star. Among the hotter of bright stars, Adhara shines with a
surface temperature of some 21,900 degrees Kelvin, which gives it
a sparkling bluish cast. From its distance of 405 light years
(second Hipparcos reduction) we
calculate a luminosity to the eye of 3500 times that of the Sun (including the effect of a six percent
dimming by interstellar dust). If at the distance of Sirius, which
dominates Canis Major, Adhara would shine at apparent magnitude -7,
7 times brighter than Venus at her most luminous. Because of the
star's high temperature, it radiates a good portion of its energy
in the invisible ultraviolet.
If that is taken into account, Adhara is actually 22,300 times more
luminous than the Sun. Indeed, if you had ultraviolet eyes, Adhara
would be the brightest star in the sky. Adhara's angular diameter
from luminosity and temperature leads to a physical radius 10.4
times solar, which is just three percent smaller than that found
from the measured angular diameter, showing that the measured
parameters are all very nearly correct. An equatorial rotation
velocity of at least 38 kilometers per second then give a rotation
period of under 14 days. Temperature and luminosity plus theory
then lead to a mass 11 to 12 times that of the Sun, which is
probably enough to make the star someday explode as a supernova (or at least to
produce a rare neon-oxygen white dwarf, most of them
being made of carbon and oxygen). Having ceased hydrogen fusion in
its core, Adhara is actually in the beginning of its death stages
(cessation of core hydrogen fusion), and is now more in its
subgiant rather than giant state. Adhara is very popular among
those who study local interstellar matter, as its simple and bright
spectrum is used to examine the stuff that lies between it and the
Sun. Seven seconds of arc away lies an 8th or 9th magnitude
(depending on whom you ask) companion that from its brightness is
probably on the line between classes A and F. The two are at least
900 Astronomical Units apart and take at least 7500 years to make
a full orbit.